In the oil drilling industry, the use of a downhole drilling motor is normally required at depths of several thousand feet. Because of the depth and difficulty involved in such drilling, it is preferable to be able to drill to completion without the necessity of `tripping out` of the hole. The failure of a motor for any reason while it is at depth will necessitate the expenditure of significant money and significant loss of time in order to trip out and repair or replace the motor. As a consequence, the durability of motors employed at the end of drill strings is extremely important.
In order for a mud motor to function, a bearing assembly must be provided in the drill string between the motor at the bottom of the drill string and the drill bit. The function of this bearing assembly is to allow the drill bit to rotate independently of the drill string. In other words, it permits rotational movement of the drill bit with respect to the fixed body of the mud motor and drill string.
It is known in the prior art to provide bearings consisting of combinations of radial load roller bearings, compressive thrust load roller bearings and extension thrust load roller bearings configured to allow rotational movement of the bottom end of the mud motor bearing assembly with respect to the top end. However, mud motor bearing assemblies incorporating or using conventional bearings for this purpose suffer from a number of deficiencies, and in particular, such mud motor bearing assemblies have suffered from mud contamination in the bearings.
While such mud motor bearing assemblies are normally comprised of oil lubricated sealed chambers containing roller bearings, over time and particularly in the environment in which they are operated, oil leaks out from the sealed chambers as the seals become worn, and mud will become introduced into the sealed chambers of the mud motor bearing assembly. With such roller bearing assemblies, it has been found that the individual rollers gradually disintegrate, and mud and small bits of metal ultimately become jammed in the assembly, causing failure of the entire mud motor bearing assembly.
In addition to the problem just described, there are also a number of other key factors to be considered when designing a downhole mud motor.
When drilling a horizontal or directional bore, or a bore which incorporates a transition from vertical to horizontal or horizontal to vertical, it is often preferable to attain the transition in the shortest possible radius. Thus, the overall length of the mud motor and mud motor bearing assembly is an important factor, and a shorter tool is better able to achieve a shorter radius.
In addition, present technology for performing measurement while drilling (MWD) allows for the accurate determination of the direction and location of the drilling tool and/or any other assemblies at the end of the drill string despite the distance from the surface or the location of the drilling rig. However, in every case, the accuracy of the MWD equipment is enhanced by locating the MWD tools as close to the drill bit as possible. Once again therefore, the length of the mud motor bearing assembly is important as the mud motor bearing assembly must be located between the motor and any such MWD equipment.
As well, overall costs associated with drilling are related among other things to the cost of manufacture of the mud motor bearing assembly and the serviceability of the mud motor bearing assembly. By reducing the cost of manufacturing mud motor bearing assemblies and increasing their serviceability, the profitability of the motors and/or the ability of the manufacturer to compete in the marketplace are augmented. As well, the complexity of the motor, including the mud motor bearing assembly, will determine the cost, speed and convenience of servicing the motor. A motor which is more complex necessarily demands more time and attention to detail and is also more likely to be subject to motor failure.